GNIRS On-Instrument Wavefront Sensor (OIWFS)

The OIWFS is an optical sub-system built by the Institute
for Astronomy (University of Hawaii). The light path is also shown in
the optical schematic.
Light from the guide field - anything in a 3 arcminute diameter
field that misses the pick-off mirror - enters the OIWFS field lens
and then passes to a collimator doublet and then a 2-axis gimbal
mirror.

The gimbal mirror can be tilted precisely
to direct light from any part of the guide field to the rest of
the OIWFS optics. The usable field is roughly 10 arcsecs diameter,
which is sufficient to acquire individual guide stars.
Light from the selected patch of sky is
reflected off the gimbal mirror, through a second doublet, and the
guide star is re-imaged on the filter wheel, which contains JHK bandpass filters as well as an ultra-wide
filter covering the entire 1.0-2.4 microns interval.

From the filter wheel, the light is focussed on the detector array; a single image of the guide star is formed there. Only
a small portion of the array is read out, allowing rapid data rates, which may permit the OIWFS to provide high-speed tip-tilt correction in addition to slower flexure and tracking corrections.